Poor / no data compression on HP U3 autoloader device

We are using Symantec Backup Exec v12.5 on a Windows 2003 server connected to a HP 1/8 G2 Autoloader (8 slot U3 device). Hardware compression is enabled on the device and this is connected SCSI port 4.

Looking at the slot properties of each tape no compression is being reported (Compression ratio is 1:1) and the amount of data being written to each tape varies between 170Gb and 308Gb.

I would expect to be able to backup a minimum of ~8 x 400Gb = 3.2Tb and with compression turned on even more but in practice we are only getting about 2Tb.